Worldwide, gastrointestinal infections and diarrheal diseases result in significant morbidity and mortality with approximately four million deaths each year. The majority of infections occur in infants and children living in developing nations. Currently, the most common etiologic agents are the rotaviruses. More recently, the role for enteric adenoviruses, astroviruses, Norwalk virus and other caliciviruses in the pathogenesis of childhood diarrheal diseases has received further clarification. Branch investigators are evaluating the role of these viruses as causes of diarrhea in a population-based cohort of Egyptian children who were monitored for a period of three years. This information should help in the development of preventive measures against infection by these viruses, including the development and future evaluation of candidate vaccines. Results showed diarrhea associated with astrovirus infection to be as common as rotavirus infection (0.19 episodes per person-year) and both viruses were found to be pathogenic. Severe dehydration was observed in 24% of the cases of rotavirus diarrhea and 17% of the cases of astrovirus diarrhea. Other than breastfeeding during infancy, no potential risk factors whose modification might control diarrheal disease due to rotavirus or astrovirus infection were identified. This observation, coupled with previous observations that diarrhea associated with these viruses occurs with comparable frequency among children in both developed and developing countries, suggest that improvements in hygiene and living conditions alone are unlikely to be sufficient preventive measures in this population. Of the 46 rotavirus strains identified during the first year of surveillance, 87% were typeable, and all were serotypes 1 and 2. The most frequent astrovirus serotype was HAstV-1 and in order of decreasing frequency HAstV-5, HAstV-8 and HAstV-3, HAstV-6, HAstV-2. In determining whether astrovirus diarrhea was associated with a reduced incidence of subsequent disease, there was evidence to suggest that HAstV-1 homotypic immunity, but not heterotypic immunity, conferred protection. Since 38% of the incidence of astrovirus diarrhea occured in infants less that 6 months of age, a candidate astrovirus vaccine would have to confer immunity very early in life.